1. Field of the Invention
The invention relates to a selfpumping hydropneumatic spring strut with internal level control, in particular for motor vehicles, with a working cylinder filled with damping medium under the pressure of at least one gas cushion arranged in a high-pressure chamber and acting as a spring, which working cylinder is subdivided into two working spaces by a working piston carried by a hollow piston rod. A piston pump which is driven by means of the spring movements and conveys damping medium out of a low-pressure chamber into the working space connected to the high-pressure chamber has a pump cylinder formed by the hollow piston rod, into which penetrates a hollow pump rod which is fastened to the working cylinder and at its front end carries a suction valve and the bore of which is connected to the low-pressure chamber. A cut-off orifice in the pump rod can be closed as a function of the position of the working piston in the working cylinder and connects the working space connected to the high-pressure chamber to a cut-off duct issuing into the low-pressure chamber. The damping medium flows through the working piston in one direction via at least one nonreturn valve, a duct running from one working space to an adjustable damping valve.
2. Description of the Related Art
Spring struts of this type are known (U.S. Pat. No. 4,993,693), in which the working cylinder is subdivided into two working spaces by a working piston carried by a piston rod and in which nonreturn valves are arranged in such a way that the flow of the damping medium runs in the same direction. This means that the damping medium constantly flows in one direction, irrespective of the direction of movement of the working piston, the damping medium in each case flowing from one working space via a duct, past an adjustable damping valve, directly into the high-pressure space or, with the piston rod extended under corresponding pressure conditions, into the other working space. Since the hollow pump rod is connected to the low-pressure space via a bore, in such a configuration both an internal level control of the system and an externally activatable, continuously adjustable damping-force control can be brought about.
Furthermore, spring struts are known (GB 2,035,511, U.S. Pat. No. 3,837,444), in which the working cylinder is subdivided into two working spaces by a working piston carried by a piston rod and in which the working piston is provided with valves for generating a damping force. In this case, a corresponding damping valve is provided for each direction of movement of the working piston. These damping valves serve for generating a specific predetermined damping force in the respective piston speed range and/or load range.